Health authorities confirmed on Wednesday that they had inspected and sampled all cooling towers and other potential Legionella sources such as decorative fountains in an area stretching from Elizabeth Street to Clarence Street, and Bathurst Street to King Street.

The blocks include the Queen Victoria Building, Pitt Street Mall, St Andrews Cathedral and the boundary of Hyde Park.

But the search is now complete, with inspectors none the wiser as to the source of the infection that has caused the hospitalisation of nine people and the death of an octogenarian.

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Legionella bacteria can cause a bacterial lung infections in people who are elderly or have other illnesses.

NSW Health director of communicable diseases Vicky Sheppeard said authorities were informed about the ninth case on Sunday, the same day that the patient was diagnosed, and a lab test confirmed a link with the current outbreak two days later.

Five patients have been discharged since contracting the disease, while three are still being treated in hospital, and the condition of the person previously reported as being in intensive care has improved.

Health officials believe that the outbreak is now over, as no new cases have been detected since March 4 and the incubation period is two to 10 days.

"In such outbreaks it's not uncommon that the source is unable to be identified, however, no further cases of Legionnaires' disease have had onset since 4 March," Dr Sheppeard said.

"Doctors, laboratories and hospitals have been actively monitoring for new cases and there has been a high level of community awareness."

The investigation will now turn to analysing - or "genetic fingerprinting" - the samples obtained from the cooling towers and patients, a process that will take longer to complete.

The disease causes coughs, chills and fever and usually affects middle-aged or older people, heavy smokers and those with chronic lung disease.

NSW Health director of communicable diseases Vicky Sheppeard said investigations into the source of the outbreak were continuing.

Authorities suspect that a contaminated water cooling tower may have blown out particles of the bacteria that were inhaled by people in Sydney's CBD.

The towers cool the water that is used in airconditioning units of large buildings.

The City of Sydney has inspected water cooling towers in an area bounded by Kent, Pitt, Druitt and Bathurst streets in the two days since Monday.

Earlier, it inspected possible sources of Legionella in the area bounded by Elizabeth, King, Clarence, Park and Druitt streets.